1. Field of the Invention
The present invention relates to a preferred transport apparatus or system to be used in the field in which an uncontacted transport of objects is required, such as a wafer transport equipment in a semiconductor production line wherein dust generation has to be avoided, a vacuum transport system wherein scattering of lubricant has to be prevented, and a transportation apparatus to be used in a high temperature atmosphere or the like. More specifically, the present invention relates to an alternating current magnetic levitation, transport system in which an object to be transported is mounted on a floating body and the floating body made of a conductive paramagnetic or nonmagnetic metallic material is floated and transported in a system over alternating current electromagnets disposed in rows.
2. Description of the Related Art
Conventionally, what is called a magnetic bearing composed of permanent magnets or electromagnets has been used as a bearing for a transport or transfer mechanism for objects (such as semiconductor wafer) in, for example, a semiconductor production process line wherein dust generation or scattering of lubricant is detested. In this system an object to be transported or supported is floated over a clearance without contacting anything by means of magnetic attraction or repulsion working between magnets or between a magnet and a magnetic material. As for the mechanisms of this kind, various researches and developments have been made and many of them have already been put in actual use.
However, among them magnetic bearings making use of electromagnets have such drawbacks as that they require control circuits for stabilizing intrinsically unstable attraction of magnets by means of electrical control although they have an advantage of small electric power consumption, or that many attachments, such as necessary gap sensors, are required. Especially, these control circuits have to be provided over the entire transport line in the case of a long distance transportation.
As a means to avoid the above-mentioned drawbacks, a necessary circuit and sensors may be mounted on a transported object. However, in this case a power source, such as batteries or the like, is required on the object and further a major problem occurs with respect to equipment that a charge or power supply system newly becomes necessary for operation over an extended period of time.
On the other hand, use of magnetic repulsion, in principle, makes the above-mentioned control devices unnecessary and has a great advantage as a transportation mechanism for a long distance in terms of the required equipment.
However, it is generally believed that a repulsion levitation mechanism composed of permanent magnets only is practically impossible, and at least one or more axes of its degrees of freedom must be stabilized by means of the above-mentioned control circuits.
On the contrary, it is theoretically confirmed that an inductive repulsion method using alternating current electromagnets can stably float and support objects by devising appropriate forms of floating bodies made of conductive materials. FIG. 11 shows such a method. In FIG. 11 (a) through (c), reference numeral 01 denotes an alternating current electromagnet, and 02 a floating body to be transported, for which a light and highly conductive material such as aluminum is suitable. An object desired to be transported is usually placed on this floating body 02.
In the composition illustrated in FIG. 11, when an alternating current flows in the aforementioned alternating current electromagnet 01, an alternating magnetic field is generated above the magnet. Because the floating body 02 is in this magnetic field, an alternating current called as an eddy current flows in the material (aluminum) of the floating body 02. The magnetic field generated due to this overcurrent is formed exactly in the repulsive direction to the magnetic field generated by the electromagnet, so that due to this repulsion a floating force indicated as Fl in the figure acts on the floating body 02.
Then the floating body 02 can be driven by means of a driving force F2 exerted thereon. This force F2 is produced by linear motor mode driving of the electromagnets disposed in rows as shown in FIG. 11 (b) and (c).
In FIG. 12, an object 03 to be transported is mounted on the floating body 02 in the apparatus shown in FIG. 11.
Described above is the transportation principle of an alternating current repulsion system. However, when this alternating current repulsion system is applied to vacuum transportation, such as in the wafer transport equipment in the semiconductor production system in which dust generation has to be avoided, it has previously been necessary to take such a measure as covering and fixing the electromagnets with resin to protect the dust generation from the electromagnets because the floating body transport passage and electromagnets are disposed in the same area. However, even if this protection measure is taken, it is deemed difficult to maintain a high grade vacuum because of a gas released from the resin when the above-mentioned electromagnets are placed in a vacuum atmosphere.
Further, the alternating current magnetic levitation transport system as shown in FIG. 11 and FIG. 12 has a problem of heat generation due to an electric current flowing in the floating body 2. As is generally known heat generation Q in a metal due to an electric current is given by EQU Q=I.sup.2 R, or Q=E.sup.2 /R
where,
Q=heat generated, PA0 I=electric current, PA0 E=electromotive force (voltage), and PA0 R=electric resistance.
Therefore, the temperature of the floating body 02 is changed when the heat equivalent to the heat generated (Q) is not removed. The temperature of the floating body 02 is elevated when the heat removal is insufficient, causing a thermal damage to the transporting object 03 or a thermal deformation and damage to the floating body 02, and normal operation becomes impossible.
As described above, the utilization of an inductive repulsion system using alternating current electromagnets for the vacuum transportation, such as the wafer transport equipment in the semiconductor production system in which dust generation is disliked, conventionally requires the countermeasures, such as, a means against dust generation or gas release from electromagnets, a means against heat generation in the electromagnets due to an electric current flowing therein, and a means against heat generation in the floating body caused by an induced electric current due to electromagnets. Especially, gas release from the electromagnets causes, in the case of a wafer transport equipment, for example, the composition of the wafer surface to deteriorate, resulting in a fatal yield reduction in the production of semiconductors while higher integration of semiconductor elements is now in progress.
Also, it is possible to cool a heated floating body by a cooled heat transfer surface. However, when transporting operation is restarted by turning on the alternating current electromagnets and the floating body is floated, the temperature of the floating body increases again, thereby the floating body needs to be cooled again by stopping the transporting operation. The smaller the frequency of operation stoppage is and the less the cooling (operation stoppage) time for cooling the floating body is, the better the operation performance of the system becomes. Therefore, further improvement of the cooling technology is required.